How Call of Cthulhu Got Big in Japan
Long story short, DnD isn't as popular and then a Call of Cthulhu fan made a popular manga/anime.
In 2009, light novel writer Manta Aisora debuted a series called Haiyore! Nyaruko-san (lit. Crawl Up! Nyaruko-san, later given the official English title Nyaruko: Crawling With Love). It was in the venerable genre of “magical girl love-comedy,” in which a girl with special powers, often from another world or dimension, falls in love with a normal Japanese guy, moves in with him, and hijinks ensue. The key here is that the magical girl, Nyaruko, is a Lovecraftian alien (reminiscent of Nyarlathotep). Later more Lovecraftian magical girls appear, to cause more chaos in the main character’s life.
The light novel was a hit, and was soon made to into a series of Flash animations from 2009 to 2011. These were enough of a success that an anime was green-lit, which began broadcast on TV Tokyo and its affiliates in April 2012. But the significant thing for our purposes is that the director, Tsuyoshi Nagasawa, was a fan and player of the Call of Cthulhu RPG. And he started liberally peppering CoC easter eggs and references into the anime. Some of it is not subtle. The opening song of the anime begins by repeating “SAN-chi pinchi!” (SAN stat in trouble!) 15 times before the verses even begin.
Interesting stuff - a world where the biggest RPG isn't DnD! Sort of makes me think of Watchmen where they imagined a world where it wasn't superheroes that dominated comics but other modes of adventure, like pirate comics.
How Call of Cthulhu Took Over the RPG Scene in Japan (RPG.net thread)